Currently much effort is being expended in developing systems for compressing video signals for both television signal transmission, and multi-media purposes, i.e. computer usage. This effort is typified by the ongoing development of a compressed video standard, which is currently identified as MPEG. (MPEG stands for "Moving Pictures Expert Group", which is a committee of the ISO, the International Organisation For Standardisation.) MPEG is a compressed video signal protocol describing a hierarchically layered motion compensated, Discrete Cosine Transformed transformed, Quantized, statistically encoded etc. signal format, a draft of which is identified as ISO document ISO-IEC/JTC1/SC29/WG11 MPEG 92/160. The MPEG protocol requires that video signal be encoded (compressed) on a frame basis in groups of frames, GOP's. The compressed signal for a GOP includes a GOP header followed by a frame header, followed by a slice header (a slice being a portion of a frame comprising, for example, 16 horizontal rows), followed by a macroblock header, (a macroblock being for example a 16.times.16 matrix of pixels) followed by blocks of pixel data, followed by the next frame header and so on.
Nominally a compression apparatus arranged to provide compressed data according to the MPEG format, receives data either on a field or frame basis, groups this data into GOP's, and performs the requisite compression processes to generate a MPEG signal. Note however, if the source material is video signal originally produced on film and converted to video via a Telecine, that is by 3:2 pulldown, one field out of five is redundant. Removal of the redundant fields from such material immediately provides a twenty percent increase in compression efficiency. Further, even if the video signal was produced by a video camera, much of this video signal may represent still images, in which case frames of data may be redundant. Elimination (at least in pan) of some of the still image redundancy will also effect an increase in compression efficiency.
In order to realize such enhancements to compression efficiency, it is necessary to identify image field redundancies and thereafter excise these fields prior to application to the compression apparatus. Further, after fields have been excised, a system of identifying excised fields must be implemented to inform the reciprocal decoding apparatus to regenerate the excised fields. The present invention is directed to apparatus for detecting redundancy in video image fields, excising ones of the redundant video image fields, and encoding the remaining data in a fashion to enable a decoding apparatus to restore the excised fields.